High current battery charging applications are numerous. One particular application is their use in recreational vehicles (RV's). A recreational vehicle is a mobile unit used for leisure activities. It typically consists of an enclosed sleeping area built on a truck or a truck-like vehicle. RVs come in many types, ranging from small, inexpensive units used for light camping to very large, expensive motor homes in which a number of people can spend long periods in a high degree of comfort. A common characteristic of RV's is that they have only a limited interior space available. This is true of even the largest RV's since they are typically filled with furniture, refrigerators, stoves, bathrooms, air conditioners, and other accessories.
Since RVs are used primarily for leisure, the comfort and convenience of their occupants is very important. Anything which reduces the comfort or convenience, such as excessive noise, interference with other accessories, excessive heating of interior spaces, or reductions in available interior space, is to be avoided.
As mentioned, an RV may have numerous accessories, which may include such items as television sets, furnace blowers, incandescent lights, radios, and stereos. Electrical power for these accessories is typically stored in twelve (12) volt direct current (VDC) lead acid batteries, similar to those found in automobiles and trucks. Since battery energy eventually becomes depleted, either because of energy used by the accessories or through internal discharge, some method of battery charging is required. The most commonly used method is to charge the batteries from an external source, such as a utility hookup at an RV campground. In RV terminology, the utility hookup is referred to as "shore power."
Shore power is typically one hundred and twenty (120) volts alternating current (VAC). Actual battery voltage will typically range from below eleven (11) volts to over fourteen (14) volts, depending on the charge on the batteries. Charging batteries from shore power requires an apparatus to convert shore power to a form suitable for battery charging. The apparatuses used are typically electrical power supplies modified for battery charging. These power supplies are classifiable into three general types, linear converters, ferroresonant converters, and switching converters.
A typical linear converter consists of a power transformer to transform the shore power to a lower AC voltage, and a rectifier to convert the AC voltage to DC. The voltage at the output of the linear converter consists of pulses of direct current; the pulse amplitude being largely dependant on the voltage amplitude of the shore power and on the current supplied to the batteries. The linear converter therefore has poor regulation with respect to changes in the shore power and in the battery charge.
Linear chargers have the advantages of simplicity and low cost. However, compared to other methods, they are heavy, bulky and inefficient. Additionally, they exhibit poor regulation against changes in shore power. If the shore power voltage drops, so does the battery charge current, if it rises, so does the battery charge current. While methods exist to correct this problem, they do so at the cost of additional circuit complexity and cost.
The pulsating nature of the output of the linear converter creates yet another drawback. Since the current supplied to the battery pulsates, it supplies a high peak current to the batteries, but a much lower average current Therefore, the conductors and the battery must be designed to accommodate high currents.
While it is easy to understand that the problem with a low charging current is the increased time it takes to charge the batteries, the problem with excessive charging currents is more complicated. Since a battery has a limited rate at which it can convert charging current into stored chemical energy, any excess charging current is converted into unrecoverable heat. Additionally, excessive charging current causes the battery electrolyte to emit excessive gases which can produce an explosive mixture. The electrolyte must be replenished or it may "dry out" by losing electrolyte. When a battery dries out the internal resistance of the battery increases, which degrades the ability of the battery to supply current to the accessories. Therefore, an optimum design of a battery charger will cause the battery to charge as rapidly as possible consistent with low degradation in battery performance.
Unfortunately for the designer, the optimum charging rate for a lead-acid battery is not a constant, but follows a "voltage compliance" curve. At a low battery voltage, denoting a deeply discharged battery, the optimum battery charging current is high; as the battery takes a charge, the optimum charging current decreases; finally when the battery is fully charged, the charge rate is very low, typically referred to as a "trickle charge." The typical linear charger does not follow the voltage compliance curve. While linear chargers can be constructed so that the output follows the voltage compliance curve, the advantages of low cost and ease of construction are lost. Additionally, the low efficiency and large size and weight of linear charges place it at a competitive disadvantage with other types at high charging currents.
One solution to poor shore power regulation is the ferroresonant converter. A ferroresonant converter typically has a ferroresonant, or constant voltage, transformer and a rectifier circuit. The ferroresonant transformer outputs a relatively constant voltage over a wide range of input voltages. Additionally, a properly designed ferroresonant converter can follow the voltage compliance requirements of a lead acid battery. These advantages make the ferroresonant converter preferable over linear converters. Ferroresonant converters are typically made with current ratings in the 40-75 ampere range. However, since a ferroresonant converter is typically only 75% efficient, at a 12 VDC, 60 ampere output the ferroresonant converter inputs 960 watts and outputs only 720 watts. The remaining 240 watts are converted to waste heat, which is highly objectionable.
Another problem with ferroresonant converters operated at high power levels is that they emit excessive audible noise. This is because a ferroresonant converter has a transformer that is typically constructed from steel lamination so that eddy current losses are reduced. These lamination are subjected to strong magnetostrictive forces during high power operation. The magnetostrictive forces cause the lamination to vibrate at the shore power frequency which creates a highly objectionable hum.
Ferroresonant transformers are extremely frequency-sensitive; they may reduce their output voltage by three percent for a frequency deviation of one percent. This creates a problem because RVs may carry engine generator sets which typically have output power frequencies which are not accurately maintained. Therefore, when a ferroresonant converter is used to charge batteries from an engine generator set, the output voltage, and therefore charging current, is highly sensitive to changes in the engine generator's frequency.
The ferroresonant converter has yet another drawback in that, like the linear converter, the output consists of pulsating currents which require batteries and conductors designed to use the high peak current.
A further problem with ferroresonant converters, as with linear chargers, is their size and weight. However, since ferroresonant converters generally operate at higher power levels than linear converters, they are larger and the drawback is more pronounced. The size and weight is a result of the material used to make the transformers, which at low frequencies is typically laminated steel.
Another approach to battery charging is the "switching" type converter. In a switching battery charger the shore power is directly rectified, and the resulting DC voltage is applied to a semiconductor switch that, on a periodic basis, switches the rectified shore power to a power output transformer. The power output transformer outputs a lower AC voltage on its secondary winding which is then rectified, smoothed to a substantially pure direct current, and sent to the batteries being charged. The output voltage is sensed and used to control the switching circuit. When a higher output voltage is required, the semiconductor switch operates with a wider pulse width, causing the output voltage to rise. When a lower output voltage is need, the semiconductor switch operates with a narrower pulse width, causing the output voltage to drop.
Some advantages of the switching battery charger are readily explained. Since the input power is rectified, the switching battery chargers is insensitive to changes in input frequency. Additionally, because a switching battery charger typically operates at a frequency much higher then the human ear can detect, the objectionable hum found in ferroresonant and linear converters is eliminated. Finally, since the output of the switching battery charger is a substantially pure direct current, the conductors and the battery do not have to withstand the high peak currents of a pulsating output.
Other advantages of switching battery chargers are not as apparent. The switching battery charger has a smaller size and lower weight for a given output current, and a much higher operating efficiency which reduces heat output. The small size and low weight result because the switching battery charger operates at a high frequency, which permits the power output transformer to be made from low weight ferrites which require less space then steel laminates. The reduced heat output results because high frequency transformers are more efficient than low frequency transformers, which means the transformer converts less input power to heat.
The switching battery charger's chief drawbacks are the high unit cost and excessive electromagnetic emissions. High cost seriously impedes the use of switching battery chargers in RV's since the demand is very price sensitive. To be competitive, the cost of a switching battery charger must not override its advantages.
Electromagnetic emissions are radiated electromagnetic energy. Electromagnetic emissions may interfere with the proper operation of sensitive devices. Three things must coexist for electromagnetic emissions to be a problem; there must be a generator of electromagnetic emissions, a propagation medium, and a victim. Switching battery chargers generate electromagnetic emissions due to the high rates of current change typically existing inside the chargers; current undergoing a high rate of change tends to generate electromagnetic emissions. The atmosphere acts as a good propagation medium and, in RV applications, the victim typically is a television set.
RV operators will generally tolerate only very low levels of television (TV) interference, essentially none. This is a problem since RV's can be located many miles from a broadcast antenna which results in a poor TV signal. Instead of the one thousand (1,000) microvolts available with a good antenna in some urban areas, the RV operator may have only two hundred (200) microvolts of signal at the TV antenna terminals. To complicate matters further, in an RV the distance between any emitter and the victim is very small. This combination of low TV signal strength and short distances between emitter and victim makes TV reception in an RV highly susceptible to electromagnetic emissions from inside the RV.
The United States Federal Communications Commission (FCC) rules for permissible radio frequency interference from computer equipment, Part 15, Sub. J, implies that electromagnetic emissions from computers should be consistent with minimal interference with TV reception in a residential urban situation. This standard is frequently used for equipment similar to switching battery chargers, even though mobile units such as RV's are exempt from its requirements. Numerous consumer complaints of switching chargers causing TV interference clearly indicate that meeting this standard is insufficient for RV applications.
The majority of electromagnetic emissions from switching battery chargers results from the fast switching of high currents in the output circuit. Current in the output circuit of a switching battery charger is typically rectangular in nature with pulse widths in the order of thirteen (13) microseconds with maximum rates of change of current in the order of two hundred (200) amperes per microsecond. While slowing the rates of change decreases the electromagnetic emissions, it increases the power dissipated in the switching transistors and diodes. This increased dissipation necessitates larger heatsinks and power transistors, causes a reduction in operating efficiency, and increases the cost, size and weight of the resulting switching battery charger.
Numerous techniques are available for reducing electromagnetic emissions. One method is to space the conductors carrying high current close together to minimize the conductor's radiating characteristics. Radiation is reduced because the loop area, a physical measure of antenna effects, is reduced which tends to reduce the electromagnetic emissions. However, Underwriter Laboratory's safety standard UL 458, a voluntary standard for recreational vehicle power converters, dictates the minimum spacing between printed circuit board conductors and between components and the chassis. Therefore, UL 458 limits the reduction in conductor spacing.
Another method of reducing electromagnetic emissions is to enclose the switching battery charger in a conductive enclosure. However, at the power levels required for high speed battery charging, this is only partially successful since openings are required for the entry and exit of cooling air. While a fine metal mesh can be used, it has a cost disadvantage because the mesh is another material to buy, and bonding it cheaply and effectively to the enclosure is a problem. Any metallic seam of the enclosure that is not completely bonded, such as between mating parts of the enclosure, can act as radiating slots. Additionally, a metal enclosure is more effective on the electric field component of the electromagnetic emissions than on the magnetic field component, which can be a problem.
As mentioned previously, in addition to the electromagnetic emissions problem, another problem associated with switching battery chargers is their overall cost and difficulty of assembly. Any acceptable switching battery charger must be easily and cheaply made and be free of assembly induced defects.
One area of difficulty in assembly switching battery chargers is the high current required, which necessitates large conductors. Since large conductors are generally difficult to work with, it is desirable to reduce the number of connections using the large conductors. One method of reducing the number of connections would be to terminate large conductors directly at the point at which the high current is used. However, wound magnetic devices such as inductors and transformers typically connect their windings to terminal lugs on the magnetic device and then attach jumper conductors between the terminal lugs and the signal destination.
Large round wires may cause problems during assembly, including the difficulties of wrapping the wire around a post, orientating the wire properly, and controlling the applied heat during soldering which contributes to temperature stress if the solder junction is overheated, and poor soldering if under heated. Additionally, if the round wire directly terminates on a printed circuit board conductor, high current densities may result from a small surface area carrying all the current, particularly in the transition from the round cross-section of the wire to the flat cross-section of the printed circuit board conductor. When solder fills a void between conductors, such as between a round wire and a hole, the solder must carry current during operation. At the high currents existing in a switching battery charger, the high current passing through the solder can produce temperature cycling and induce undesired metallurgical changes in the solder, such as selective crystallization of alloying elements.
The present invention solves these and other problems associated with switching battery chargers.